1. Field of the Invention
This invention relates to a digital display micrometer gauge for measuring dimensions of a workpiece, including length and thickness, through displacement of a spindle.
2. Description of Related Art
The micrometer gauge has been conventionally used as a gauge for measuring dimensions of a workpiece through displacement of a spindle. Since it is easy to handle, this type of gauge is used by many machinists. As described in U.S. Pat. Nos. 4,550,507 and 4,578,868, the micrometer gauge generally comprises a U-shaped main frame, to which an inner sleeve is secured. A female screw is formed in the inner sleeve by precision machining, and a male screw of a spindle, likewise formed by precision machining, is engaged with this female screw. The spindle rotates through a thimble connected thereto, and, by reading the amount of this rotation, the workpiece is measured.
In this conventional micrometer gauge as described above, the rotation of the thimble is transmitted to the spindle through the male and female screws. The screw pitch is generally small, approximately 0.5 mm for example, so that the spindle cannot be displaced at high speed. This results in a rather poor operational efficiency, particularly when measurement is repeatedly conducted. A high speed displacement of the spindle might be ensured by roughening the screw pitch. However, that would result in deterioration in the measuring accuracy of the micrometer.
Micrometer gauges designed to solve the above problems are proposed in U.S. Pat. No 4,485,556 and Japanese Utility Model Publication No. 3-4883. In these gauges, the spindle is supported by the main frame at two positions of its middle section through the intermediation of stationary bearings. The spindle is displaced by a spindle displacement mechanism. The spindle displacement mechanism comprises a sleeve engaged with the main frame such that the sleeve is rotatable but not movable in the axial direction (dimension) of the spindle. A spiral groove is formed on the inner side of this sleeve and has a relatively large pitch. A spindle engagement section engages this spiral groove, and a spindle rotation preventing means controls the rotation of the spindle relative to the main frame. Due to this mechanism, high speed displacement of the spindle is ensured.
To ensure high measurement accuracy in digital display, the conventional gauge disclosed in Japanese Utility Model Publication No. 3-4883 includes, in addition to the above-described driving mechanism, an encoder. The encoder has a main scale arranged in the middle section of the spindle through the intermediation of an arm member such that the main scale extends parallel to the axial dimension of the spindle and an index scale provided on the main frame. The displacement amount of the spindle is detected in digital form by this encoder. In this type of conventional gauge, in which the spindle is directly supported by two bearings secured to the main frame, it is difficult for the main scale to be directly attached to the spindle. That is, when the main scale is directly secured to the spindle, it is necessary for the spindle to be considerably long. Otherwise, the main scale would hit against one of the bearings secured to the main frame when the spindle is moved away from the anvil. Thus, the size of the gauge would have to be rather large. That is why the main scale of the above-described conventional gauge is supported at a position offset from the axis of the spindle by a predetermined distance and extends parallel to the spindle.
However, this structure tends to generate measuring errors due to Abbe's principle. Furthermore, in this conventional gauge, the main scale supported by the arm member may oscillate when the entire gauge oscillates. So, the relative distance between the scale and the position transducer is subject to fluctuations, thereby causing measuring errors.